APS and PPS are x-y addressable solid state imagers wherein each pixel contains both a photosensing element and a select element. For APS, each pixel also contains at least one other active circuit component. In both APS and PPS, incident illumination is converted to a signal (either a voltage or current signal). The signal represents the amount of light incident upon a pixel photosite. This signal is typically readout one row at time, and the signals for a given row are stored temporarily in a circuit associated with each column of the image sensor. This column circuit is typically constructed to fit into the size or pitch of the pixel.
For many digital imaging applications, it is desirable to have a large number of pixels in a given size image sensor in order to increase the resolution of the image sensor. As the resolution requirement increases, the required pixel size decreases. As the pixel size decreases, several image sensor design and performance disadvantages are encountered. First, it becomes increasingly more difficult to construct a low noise column storage and readout circuit. Second, smaller pixels have lower sensitivity and can provide inadequate signal levels for low levels of illumination. Third, for a large number of pixels, the readout time will become longer. In many cases, a camera is required to produce video as well as still images.
Typically the video rate desired is 30 frames per second. Prior art APS and PPS sensors have accomplished video rate data from large resolution sensors by windowing or sub-sampling of the image array using the x-y addressability feature of APS and PPS sensors. While this approach provides video rate data, it does so by selective readout of the small pixels and still has poor image quality in low light level environments, and produces aliasing image artifacts.
Some APS and PPS sensors also include on sensor white balance by placing a programmable gain amplifier PGA in the readout path, which gain can change at a pixel data rate. For high resolution sensors, this has the disadvantage of requiring higher performance PGAs.
From the foregoing discussion it should be apparent that there remains a need within the prior art for a high resolution, small pixel device that provides high readout rate, variable resolution while retaining low noise and high sensitivity.